With the increasing scarcity and expense of fossil fuels, the development of alternative fuel sources and heating and cooling systems has become desirable. One possible alternative is the conversion of solar energy. Many proposals and designs have been made for collecting solar energy, most of which have generally comprised exposing a liquid to sunlight to directly increase the temperature of the liquid sufficiently to be able to extract heat therefrom. The development of an economical and efficient means of heating or cooling buildings with solar energy is considered an important requirement for the implementation of solar energy as an alternative to fossil energy sources for producing thermal conform in buildings. A major objective has been to collect as much sun and sky radiation as possible, at the highest attainable temperature, for the lowest possible investment in labor and materials.
As outlined in UTILIZATION OF SUN AND SKY RADIATION FOR HEATING AND COOLING BUILDINGS by John I. Yellott in ASHRAE JOURNAL, December, 1973, pages 31-41, solar radiation can be collected by a flat plate collector which can use both direct and diffuse radiation, or a concentrating device, which can use only the direct rays of the sun. A flat plate collector generally consists of five components: (1) glazing, which may be one or more sheets of glass or plastic; (2) tubes or fins for conducting or directing the heat transfer fluid from the inlet duct or heater; (3) a plate, generally metallic; (4) insulation; and (5) a container or casing which protects the components from dust and moisture. Flat plate collectors are generally large and cumbersom and are placed facing south in a tilted arrangement at some distance from the building they are to heat, or on the roof of the building. Either arrangement is undesirable from both an exthetic or structural viewpoint.
Temperatures higher than those possible with flat plate collectors are attainable if a large amount of solar radiation is concentrated upon a relatively small collection area. As pointed out by Yellot in the noted publication, paraboloidal concentrators can attain extremely high temperatures, but they require very accurate tracking systems and they can use only the direct rays of the sun, since diffuse radiation cannot be concentrated. The publication also notes that the principal use of concentrating collectors in the past has been in the production of steam or high temperature fluids for use in refrigeration or power generation. The higher cost and added mechanical complexity of collectors which must follow the sun, and their inability to function at all on cloudy or overcast days are pointed out as disadvantages.
POPULAR MECHANICS edition of September, 1975, describes a number of concentrator type collectors. In the Northrup collector which is described, a curved Fresnel lens focuses intensified sun heat onto an absorber tube located at the bottom of a steel trough. An array of up to forty collectors swivels to track the sun. Another solar concentrating system to increase the temperature at which the solar energy is collected is described in J. VAC. SCI. TECHNOL.; Vol. 12, No. 1, Jan./Feb., 1975. In the system described, a parabolic trough concentrates the solar energy onto a thermal transfer pipe at the focus of the system. The pipe is surrounded by a glass cylinder which is evacuated to reduce convestive and conductive losses. The parabolic reflector must track the sun to maintain the image on the thermal transfer pipe.
Collector plates, or concentrating devices assembled in an array, take up considerable space and must be located at some distance from the building they are to service, or on the roof thereof as previously noted, making such arrangements undesirable from a number of viewpoints. Other solar energy conversion arrangements capable of being incorporated into existing structures or new buildings in an effective and esthetic manner and therefor desirable.